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THE ACID BESSEMER PEOCESS.                               103
from a paper by Prof. Howe.*. The results on the'slags are not accurate, for it is impossible to take a true sample of converter slag, on account of its viscosity. An attempt to work out the weight of the cinder at different periods of the blow showed that there were considerable discrepancies; the combustion of the metalloids is not in proportion to the amount of air given as entering the vessel, while the total oxygen in the recorded volume of air is twice the amount needed for the silicon, manganese and carbon. Notwithstanding these errors, the table represents the chemical operations in the vessel. The presence of phosphorus in the slag is attributed by Prof. Howe to shot mechanically held. This is hardly the whole story, for I have found that acid open-hearth slag with 50 per cent. Si02 may carry 0.04 per cent, of phosphorus, and this must arise, in part at least, from an absorption of phosphorus by oxide of iron. The failure of the silica to break up the phosphate of iron may be explained by the persistence with which traces of elements refuse to be eliminated under conditions which suffice for the removal of all but an inconsiderable proportion. I have elsewheret dwelt upon this fact.
SEC. Vic.—Variations due to different contents of silicon.—With a low initial heat, the elimination of silicon is almost complete before the carbon is seriously affected, but there is a critical temperature where the relative affinities of silicon and carbon for oxygen are reversed, and, when this is attained, no matter at what stage of the operation, the silicon immediately ceases to have preference, and the carbon seizes the entire supply of oxygen. This continues until the carbon is reduced to about .03 per cent. If the metal has contained silicon during the burning of carbon, owing to an excessively high temperature, the blowing may be kept up after the drop of the carbon flame and the silicon will be oxidized in preference to iron, but in ordinary practice silicon is eliminated early in the operation, for scrap is added to the charge in sufficient quantity to utilize the excess of heat. The same cooling effect may be attained by the injection of steam into the air supply.
It has been the practice at many foreign works to have the pig-iron at a high temperature in the manufacture of rail steel, and blow "hot" to produce a decarburized metal containing silicon. The
* Notes on the Bessemer Process.   Journal I. and S. I., Vol. II, 1890, p. 102. t The Open-Hearth Process,   Trans. A. I. M. E., Vol. XXII, p. 462.